The present invention relates to an electric discharge type ozone generator and an electric discharge cell there of for generating ozone (03) which is used for water sterilization, oxidation bleaching requiring a high degree of oxidation, semiconductor manufacturing processes, and so on.
Conventional electric discharge cells for a discharge type ozone generator are shown in FIGS. 9 to 11. Each of the electric discharge cells of FIGS. 9 to 11 comprises a high-voltage electrode 13, a low-voltage earth electrode 14, and a dielectric 15 and a space 16 for discharge provided between the electrodes 13 and 14. The high-voltage electrode 13 is connected to the high-voltage side of an alternating-current high-voltage power source 17. The earth and the earth electrode 14 are connected to the low-voltage side of the alternating-current high-voltage power source 17.
FIG. 9 is a general side view of the electric discharge cell. FIG. 10 is an enlarged cross-sectional view, taken along the line E—E in FIG. 9. In the electric discharge cell of FIGS. 9 and 10, the high-voltage electrode 13 is provided along an inner circumferential surface of the dielectric 15 (which consists of a glass tube), and the earth electrode 14 in a cylindrical form is arranged so as to face an outer circumferential surface of the glass tube. The dielectric 15 is disposed between the high-voltage electrode 13 and the earth electrode 14. A material gas containing oxygen flows through the space 16 for discharge, and a part of the oxygen is converted to ozone.
FIG. 11 is a general cross-sectional view of an essential part of a gaseous discharge reaction apparatus disclosed in JP-B2-6-51113 (Examined Japanese Patent Publication, Kokoku). The high-voltage electrode 13, which is connected to the high-voltage side of the alternating current high-voltage power source 17, is arranged to face the earth electrode 14 which is covered by the dielectric 15. An electrode surface of the high-voltage electrode 13 facing the earth electrode 14 includes a number of grooves extending parallel to each other. As shown in FIG. 11, each of the grooves is a so-called “trench groove,” which is formed by two planes intersecting at a substantially right angle. The grooves as viewed in section along a longitudinal direction are saw-toothed. In the apparatus of FIG. 11, the material gas flows in a direction parallel to the trench grooves or in a longitudinal direction of the grooves, that is, in a direction perpendicular to the paper of FIG. 12.
FIG. 12 is an enlarged view of a trench groove 16, showing a field of discharge in the apparatus of JP-B2-6-5113. A field Q of discharge indicated by grid-like hatching, which is called a creeping discharge field, is generated in a region in the vicinity of an indentation 11 of the groove. The remaining region in the groove is occupied by a field P of discharge indicated by diagonal hatching, which is called a silent discharge field. The material gas passes through the fields P and Q of discharge at a high density, to thereby conduct an efficient discharge reaction.
A uniform discharging gap, such as that shown in FIGS. 9 and 10, is suitable for effecting flow of the material gas in a large volume and generating a large amount of ozone. However, such a uniform discharging gap is not suitable for generating a gas containing a high concentration of ozone. On the other hand, in a discharging gap formed by a trench groove such as that shown in FIGS. 11 and 12, a gas containing a high concentration of ozone can be generated as compared to the above-mentioned uniform discharging gap. However, when a flow rate of the material gas is reduced to further increase the concentration of ozone, the material gas tends to flow into a region close to the indentation 11 of the trench groove, which imparts only a small resistance to the flowing material gas. Thus, it is difficult to make the material gas flow at a ridge portion 12 of the trench groove, where a high density of discharge can be obtained and a gas containing a high concentration of ozone can be easily generated. Therefore, in the discharging gap formed by the trench groove, a concentration of ozone as high as about 10 vol % can be obtained, but it is difficult to achieve any higher concentrations.
JP-B2-2983153 (Japanese Patent Publication) discloses various structures of high-concentration ozone generators. In the ozone generator of this patent, the gas pressure in the space for discharge is set to 1 atm or more, and the length of a discharging gap in the space for discharge is set to 0.4 mm or less. Further, a plurality of members for maintaining a predetermined distance between the electrodes are distributed separately over the entire electrode surface, and a stress buffering plate is arranged between two ozone generators arranged in a stacked configuration for maintaining the predetermined length of the discharging gap in the space for discharge.
JP-A-9-504772 (Japanese National Publication of PCT, Kohyo) discloses a lightweight and compact ozone generating cell for generating a gas containing a high concentration of ozone. The cell of this publication comprises a high-voltage assembly comprising a high-voltage electrode, a low-voltage assembly comprising a low-voltage electrode, a barrier dielectric means for defining a discharging region for generating an ozone-containing gas between the electrodes, and a weld seal portion for connecting the assemblies. A permanently sealed chamber is formed between the assemblies to include the discharging region.
In recent years, the range of applications of ozone (03) which is used as a cleaning agent for oxidation sterilization has been expanding. However, as mentioned above, the concentration of ozone generated by ozone generators is generally low, which has inhibited the use of the gas in industry. Various methods for increasing the concentration of ozone produced during its manufacture have been proposed, such as the use of liquidation and adsorption, and some of them have been put into practice. However, apparatuses used in these methods are complicated and expensive, which highly limits their use.